1. Field of the Invention
The present invention relates to a technique of generating map data required for drawing or displaying a map.
2. Description of the Related Art
Digital map data are used for display of maps on the screen of a computer or a navigation system. The digital map data consist of diverse polygon data representing roads, railways, constructions, and variety of other features. Various techniques have been proposed to automatically generate such polygon data, especially road polygon data for the relieved load on generation of the digital map data.
For example, a technique disclosed in Japanese Patent Laid-Open Gazette No. H06-83937 utilizes path finding data or network data consisting of nodes and links and automatically generates road polygons of preset widths from link data. Another proposed technique adjusts a polygonal line connection between adjacent road polygons to smooth arc geometry or adjusts a connection of adjacent road polygons with different widths to tapered geometry. Another technique disclosed in Japanese Patent Laid-Open Gazette No. 2002-333829 generates road polygons of preset widths from network data and subsequently generates edge line data of the road polygons, based on the spatial relation of the road polygons, for example, based on determination of whether any generated road polygon represents an elevated road.
Some of major requirements for the digital map data are the improved appearance of a resulting map and the relieved load on generation of digital map data. Roads may be color coded according to their road classes, for example, national highways or prefectural roads. The conventional color code technique may, however, cause some awkwardness in display of road polygons of certain geometry at an intersection between a national highway and a prefectural road. This damages the appearance of a resulting map. The operator's manual adjustment of such awkward display for the improved map appearance undesirably requires the heavy load for generation of map data.
When two practically parallel roads are located close to each other, for example, when an up lane and a down lane are formed by separate links or when a ground road runs parallel to an elevated road, the conventional automatic polygon generation technique may cause an overlap of two road polygons representing these two practically parallel roads. In order to avoid the awkwardness in display of map drawing, the typical procedure gives priority to one of the overlap polygons and eliminates the edge lines of the other overlap polygon. Uniform priority setting and uniform elimination of edge lines between the two practically parallel roads are essential for leaving the continuous edge lines of the overlap polygons in the map. Such elimination of the edge lines is manually specified by the operator and thus requires the extremely heavy load. In order to eliminate such drawbacks of the prior art techniques, the object of the present invention is to provide a technique of automatically generating road polygons with less awkwardness in display of map drawing.